Cambo, I would put porn addiction as destructive as gambling, alcohol, drugs, etc, and certainly more prevalent.
It has been proven to rewire neural pathways. I believe it contributes to marital break ups, suicides and too early sexualisation of our children.
With it so easily available nowadays on pc's and smartphones I dread to think what our kids are looking at in the playground.
Whilst some may be less 'harmful, the predisposition in many is that normal stuff soon becomes mudane, driving the user to looking at more extreme and sinister stuff. This may not be the rule for some people, but for others I believe to be true.